Exciter frequency ability system

ABSTRACT

An exciter frequency agility system is disclosed which provides broad separation of carrier sidebands, enhancing microwave system agility while eliminating the need for tunable filters. Preselected, precisely controlled frequencies are combined to provide a high megahertz stable oscillator frequency. This stable frequency is split and directed along three distinct channels for selected combination with an output signal of a variable high frequency generator. In combining the stable frequency with the generated variable frequency, the channels are isolated from adjacent channels and recombined to provide two synchronized output frequency ranges. One frequency is coupled to the receiver front end, the other is transmitted toward the target. The two frequency ranges or bands are maintained a fixed high frequency apart while being variable within their respective ranges. The two signals are combined when the transmitted energy is reflected back to the receiver. Frequency spacing is of such magnitude that the received reflected energy, when recombined, is easily filtered with a bandpass filter to obtain intelligence therefrom.

United States Patent 1191 Barley et al.

[ 1 EXCITER FREQUENCY ABILITY SYSTEM [75] Inventors: Thomas A. Barley;Gustaf J. Rast,

Jr., both of Huntsville, Ala.

[73] Assignee: The United States of America as represented by theSecretary of the Army, Washington, DC.

1221 Filed: Dec. 21, 1972 211 Appl. No.: 317,320

[52] U.S. Cl 325/444, 325/20, 331/22, 331/38, 343/175 [51] Int. Cl. H04bl/26, H04b l/30 [58] Field of Search 325/19, 20, 431, 432, 436, 325/442,444; 331/2, 22, 37, 38; 343/l7.l R,

Primary ExaminerBenedict V. Safourek Assistant Examiner-Marc E.Bookbinder Attorney, Agent, or Firm-Edward J. Kelly; Herbert Berl; JackW. Voigt 1 1 Apr. 9, 1974.

[57] ABSTRACT An exciter frequency agility system is disclosed whichprovides broad separation of carrier sidebands, enhancing microwavesystem agility while eliminating the need for tunable filters.Preselected, precisely controlled frequencies are combined to provide ahigh megahertz stable oscillator frequency. This stable frequency issplit and directed along three distinct channels for selectedcombination with an output signal of a variable high frequencygenerator. ln combining the stable frequency with the generated variablefrequency, the channels are isolated from adjacent channels andrecombined to provide two synchronized output frequency ranges. Onefrequency is coupled to the receiver front end, the other is transmittedtoward the target. The two frequency ranges or bands are maintained afixed high frequency apart while being variable within their respectiveranges. The two signals are combined when the transmitted energy isreflected back to the receiver. Frequency spacing is of such magnitudethat the received reflected energy, when recombined, is easily filteredwith a bandpass filter to obtain intelligence therefrom.

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06.3402 6 mwhzu w? OE EXCI'IER FREQUENCY ABILITY SYSTEM BACKGROUND OFTHE INVENTION Frequency agility in a radar occurs when a radar changestransmission frequency rapidly. The ability to rapidly change frequencyintroduces tuning problems in transmitter and receiver filter circuits.At the higher frequencies (for example, 300 megahertz and up) there islittle separation of intelligence carrying sidebands and carrierfrequencies. When filtering or selecting one sideband, turning becomesdifficult. A minimum shift in the transmitted signal can result in asubstantial shift as seen by the finely tuned filters in the receiver,making problematical the selection of a desired sideband and suppressionof the unused sideband. Thus, stringent and unrealistic demands uponfilter design results in forcing selection of a filter which providesthe best compromise.

SUMMARY OF THE INVENTION In the exciter frequency agility system,frequencies are selected and mixed for exclusive selection of thedesired signal spectrum range. Undesired sidebands resulting from themixing process are sufficiently displaced from the desired signal to beremoved by conventional fixed tuned bandpass filters. The mixing andfiltering process is arranged to allow a fixed frequency offset to existbetween the transmitter and receiver local oscillator frequencies. Byselective filtering, amplifying, and routing of the signals, thereceiver and transmitter channels are segregated. A stable oscillatorserves as a coherent oscillator source which is inserted into therespective channels. The respective channels are separated or offset bya predetermined fixed frequency difference obtained when the coherentoscillator frequency is mixed with other selected frequencies. Thestable oscillator frequency is directed into a third channel and used asa local oscillator source for mixing with a selectable, programmablefrequency source. The programmable mixed output is carefully isolated,split, and mixed with the respective transmitter and receiver channelfrequencies. Therefore, the receiver and transmitter channels are alwaysseparated by a fixed high frequency band. This system allows sidebandrejection with a fixed tuned broad band filter due to the separation,thereby eliminating any need for tunable filters.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a generalized block diagramof a preferred embodiment of the invention.

FIG. 2 is a single line schematic of the invention showing the changesin frequency through the system.

FIG. 3 is a block diagram of the up-converter network of FIG. 1

FIG. 4 is a block diagram of the directional circuit of FIG. 1.

FIG. 5 is a general block diagram of the isolation and amplifier networkof FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT In an exciter frequency agilitysystem, high frequencies are selectively mixed to provide substantialspacing between interferring frequencies in the spectrum. This allowseconomical use of simple bandpass filters to pass the desiredfrequencies while blocking adjacent interference signals. No tunablefiltering is needed to restrict unwanted signals or spurious radiation.

Referring now to the drawings wherein like numbers represent like partsin the several figures, FIG. 1 discloses apparatus representing apreferred embodiment of the exciter frequency agility system. In FIG. 1,an upconverter has respective fixed input frequencies f1, f2, f3 and f4from a coherent oscillator source for selective mixing therein toprovide output frequencies f7 and f8. Frequencies f7 and f8 are combinedin coherent mixer 200 to provide a summed output f9 which functions as astable coherent oscillator signal. F9 is coupled as an input to a signaldirection circuit 300 which has plural output branches or channels forcoupling coherent signal 19 to separate channels of an isolation andamplifier network 400. Isolation network 400 has an additional input ofhigh frequency f4 coupled from the input of f4 to converter 100. Afrequency synthesizer 410 or other high frequency generating means forproviding a selectable or programmable radio frequency source providesan output band of frequencies around frequency fl0. This frequency band,referred to as 110, is capable of being swept rapidly between extremesfor continuous control as is well known in the art. F10 is coupled as avariable input signal to isolation network 400. The sum and differencecombination of respective input frequencies in isolation network 400results in output frequencies f13 and fl4 from the exciter to usingcircuitry. In a radar having frequency agility, the frequency fl3, whichvaries with f.I0, is coupled to the receiver amplifier channel of theradar for combination with target reflected energy of fl4. Frequencyfl4, also variable as fl0 varies, remains a fixed frequency apart fromfl3 due to the mixing process and is directed to the radar transmitterintermediate power amplifier for radiation toward a target. When thesetwo signals are combined in the receiver circuit, the differencefrequency between the two is obtained for further processing.

In FIG. 2 a simplified single line diagram of the frequency agilitysystem discloses the order of combination of the frequencies. Inconverter 100, f1 and f2 are combined in a mixer 120, producing the sumfrequency f5 =fl +f2. Frequenciesf3 and f5 are then summed in a mixer140, providing an output f6. Input frequency f4 is then summed with f6in mixer 160, producing output f7. Frequency f4 is also coupled througha homodyne multiplier wherein thefrequency is mixed with itself toproduce the fourth harmonic of f4 which is selected as f8. Operation ofthe homodyne multiplier is as disclosed in a copending application Ser.No. 289,025, filed Sept. 14, 1972 by Barley et al. and entitled HomodyneMultiplier. Frequencies f7 and 18 are summed in coherent mixer 200,providing the stable coherent oscillator frequency 19 which is coupledto director circuit 300. Directional couplers within the directorcircuit splitj9 into channels 1, 2 and 3 for coupling into isolation andamplifier network 400. In channel 1, f9 is coupled to a mixer 430 whereit is mixed with f4, one of the input frequencies of converter 100,producing a difference frequency output f9 f4 fl 1. In channel 3, f9 iscoupled as an input to mixer 450 wherein it is summed with synthesizeroutput f10 to provide a varying output frequency band around fl2. Theoutput signal, j12, is coupled into mixers 470 and 490. In mixer 470,fl2 is summed-with'difference frequency fll to provide the summed outputsignal 114.

In channel 2, f9 is coupled to mixer 490 and summed with fl2 to provideoutput signal fl3.

FIG. 3 more particularly shows the circuitry of upconverter 100. Theoutput of mixer 120 (I5) is serially connected through an attenuator pad124, a fixedtuned bandpass filter 126, an amplifier 128 and anotherattenuator pad 124 to a first or high decibel (db) input of mixer 140.Attenuators 124, filter 126, and amplifier 128 form a typicalfixed-tuned filter-amplifier circuit 130. Frequency f3 is coupledthrough an attenuator 132 to the second or low input of mixer 140. Theoutput of mixer 140 (f6) is connected through a filteramplifier circuit130 to the low input of mixer 160. Frequency f4 is connected throughattenuator pad 134 and a fixed-tuned bandpass filter 136 to the input ofa hybrid 150. Hybrid 150 splits f4 into separate paths, one output beingconnected through a filter-amplifier circuit 130 to the high input ofmixer 160 and the other output being coupled to the input stage of anamplifier 152. The output of amplifier 152 is connected to a homodynemultiplier 180 which splits and mixes f4 with itself to produce anoutput spectrum therefrom. A filter-amplifier circuit 130 is connectedto the output of multiplier 180 and selects f8 from adjacent harmonics.Frequency f8 is coupled with the low input of coherent mixer 200. Theoutput of mixer 160 is connected through filter-amplifier circuit 130 tothe high input of mixer 200.

Similar filter-amplifier circuits 130 in up-converter 100 are identicalin function but may vary in components and the level of frequencyresponse to provide predetermined db levels to the mixers. For example,the amplifier may have fixed-tuned filters on both the input and outputstages and the attenuators would not be tuned to the same decibel level.Hence, filteramplifiers 130 are so designated because of the similarcircuit function and need not be the same in structure.

As shown in FIG. 4 the output, f9, of coherent mixer 200 is coupledthrough a fixed-tuned isolator-amplifier circuit 320 to the input of adirectional coupler 340. Isolator-amplifier circuit 320, similar tocircuit 130, comprises a serially connected attenuator pad 322,fixed-tuned bandpass filter 324, isolator 326, amplifier 328, andanother isolator 326. Directional coupler 340 has a first output coupledas channel 3 through an isolator-amplifier circuit 320 to the high inputof mixer 450 (FIG. 5). Another output of coupler 340 is connectedthrough an isolator circuit to amplifier 342 wherein )9 is amplified andconnected to a second directional coupler 350. A channel 1 output ofcoupler 350 is connected as a low input through fixed-tuned filter 420to mixer 430 shown in FIG. 5. Similarly, channel 2 output of coupler 350is connected through an isolatoramplifier circuit 320 to the high inputof the mixer 490 in FIG. 5. Direction circuit 300 and isolationamplifier network 400 include somewhat similar isolationamplifiercircuits 320. These amplifier circuits can vary in structure by havingmore than one filter or attenuator pad for example and only oneisolator, depending on the decibel level required for the particularfrequency passing therethrough.

As further seen in FIG. 5, f4 is also coupled to the high input of mixer430 for combination with )9. The output of mixer 430, fll, is connectedthrough an isolator-amplifier circuit 320 to the high input of mixer470. The output signal (flO) from frequency synthesizer 410 is connectedas the low input for mixer 450 and is combined with 19 to produce outputfrequency fl2. The output of mixer 450 is connected through anisolator-amplifier circuit 320 to the input of hybrid 460. Hybrid 460splits fl2 into first and second outputs, the first output beingconnected through isolator and fixed-tuned filter combination 462 to thelow input of mixer 490 where it is combined with f9 to provide theoutput variable frequency fl3, the other output of hybrid 460 beingconnected through a fixed-tuned filter and isolator circuit 464 to thelow input of mixer 470 where it is combined with fll to produce variableoutput frequency f14. These output frequencies are connected throughrespective fixed-tuned bandpass filters 472 and 492 to using circuitry,allowing efficient sideband rejection without the need of turnablefilters.

System operation can be across the spectrum of microwave frequenciesdepending on the particular range of output frequencies desired.Directional circuit 300 and isolation and amplifier network 400 includeappropriate isolation stages to segregate respective receiver andtransmitter output channels (channels 1 and 2). Because of frequencyspacing, filters for respective channels need not be tunable and may befixed broad, or narrow bandpass filters for the frequency beingseparated. The channel serving the receiver is amplified, filtered andisolated to feed the radar receiver mixer circuit as a local oscillatorfrequency source. The channel for the transmitter is split off from thereceiver channel, filtered, and mixed with the f4 frequency to providean offset, difference, frequency for separating the transmitter andreceiver channels by f4. This difference frequency provides the drivingsignal for the transmitter mixer circuit. The third channel being mixedwith the programmable frequency source (synthesizer 410) produces adesired sideband range which is filtered to eliminate undesired mixerby-products and the power level is kept low to reduce undesirednonlinear effects from polluting the signal. The frequency selectablerange, fl2, is then split, isolated, and mixed with respectivetransmitter and receiver channels. Thus, two variable frequency outputsare provided which always remain separated by the fixed frequency f4.

No tunable filters are required to reject sideband signals since asimple rf bandpass filter of fixed frequency will remove unused orundesired components. During mixing, the signal which is to dominate theoutput spectrum is coupled as a relatively high decibel level into themixer first or high input while the least dominate signal is coupled ata lower decibel level as the mixer low input. This dictates the numberof amplifier stages and level of attenuation which may be providedbetween mixers as well as controlling the frequencies present inprogressive stages of the system.

Obviously many modifications and variations of the exciter frequencyagility system are possible in the light of the foregoing disclosure. Itis therefore understood that with the scope of the appended claims theinvention may be practiced otherwise than as specifically describedherein.

We claim:

1. An exciter frequency agility system comprising: an up-converternetwork having a plurality of input frequencies and first and secondfrequency outputs; a coherent frequency mixer for combining the outputfrequencies of said up-converter and providing a coherent frequencyoutput; a direction circuit coupled for receiving the coherent output ofsaid frequency mixer and having a plurality of common coherent frequencyoutputs; an isolation and amplifier network having a plurality offrequency inputs and first and second outputs, said outputs havingsimultaneously varying frequencies thereon which remain spaced apart bya fixed frequency difference; said plurality of direction circuitcoherent frequency outputs being coupled to respective isolation networkinputs; a frequency synthesizer having a high frequency variable outputcoupled to a first input of said isolation network; and a second inputof said isolation network being coupled in common with one of saidplural inputs of said converter network.

2. An exciter frequency agility system as set forth in claim 1 whereinsaid up-converter network comprises first, second and third seriescoupled mixers, each of said mixers having first and second inputs andan output; the output of said first mixer being coupled as a first inputto said second mixer, the output of said second mixer being coupled as asecond input to said third mixer; the output of said third mixer beingcoupled as a first input to said coherent frequency mixer, said firstmixer being coupled to receiver first and second frequencies of saidconverter plurality of input frequencies, said second mixer having thesecond input coupled to receive a third frequency of said converterplural input frequencies, said third mixer having the first inputcoupled to receive a fourth frequency of said converter plural inputfrequencies; and a multiplier coupled between a second input of saidcoherent frequency mixer and said fourth input frequency of saidconverter.

3. An exciter frequency agility system as set forth in claim 2 andfurther comprising a hybrid having an input responsive to said fourthinput frequency of said converter, said hybrid having a first outputcoupled to the first input of said third mixer for coupling said fourthfrequency thereto; and wherein said multiplier is a homodyne multiplierhaving an input coupled to a second output of said hybrid, an output ofsaid multiplier being coupled to the second input of said coherent mixerfor producing the output spectrum of said converter fourth inputfrequency.

4. An exciter frequency agility system as set forth in claim 3 andfurther comprising a fixed-tuned filteramplifier circuit in each of therespective inputs for said coherent mixer and said third mixer and inthe first input of said second mixer for filtering and controlling inputsignal levels coupled to said mixer.

5. A frequency agility system as set forth in claim 4 wherein saidisolator and amplifier network further comprises third, fourth and fifthinputs from said common coherent frequency; fourth, fifth, sixth andseventh mixers for selectively combining said network input frequenciesto provide first and second spaced apart variable output frequencies;said fourth mixer having the output coupled as a first input to saidfifth mixer and having first and second inputs respectively coupled tosaid fourth frequency as said second input to said isolation network andto said third input for providing a fixed output frequency differencesignal to said fifth mixer, said sixth mixer having a first inputcoupled to said coherent fourth input, said seventh mixer being coupledto said first and fifth network inputs for receiving and mixing saidsynthesizer variable output signal and said coherent frequency; a hybridjunction responsive to the output of said seventh mixer for splittingand coupling said variable output signal as respective second inputs tosaid fifth and sixth mixers; and first and second fixed-tuned bandpassfilters coupled respectively to the output of said fifth mixer and saidsixth mixer for coupling said spaced apart, variable output frequenciestherefrom.

6. A frequency agility system as set forth in claim 5 and furthercomprising respective fixed-tuned filters and isolators connectedbetween said hybrid outputs and said fifth and sixth mixer inputs forselectively isolating the variable frequency input thereto.

7. In an exciter frequency agility system, the method of generating avariable output signal for transmission which is selectively spacedapart from another variable output signal for combining with the firstvariable signal to provide a known difference frequency therebetween andcomprising the steps of: mixing first and second high frequencies toobtain a third frequency sum output; mixing a fourth high frequency withthe third frequency to provide a fifth frequency sum output; mixing asixth high frequency with the fifth frequency for providing a seventhhigh frequency sum output; homodyne multiplying said sixth frequency forproducing the fourth harmonic of said sixth frequency; mixing the fourthharmonic of said homodyne sixth frequency with said seventh frequencyfor providing an eighth frequency, stable, coherent output signal; andselectively mixing individual ones of said sixth, eighth, and ninthfrequencies with mixed combinations of said sixth, eighth and ninthfrequencies to provide and selectively spaced apart variable outputfrequencies.

8. The method as set forth in claim 7 and wherein said selective mixingcomprises the steps of: mixing said eighth frequency with said sixthfrequency for providing a tenth frequency output difference signal,mixing said eighth frequency with said ninth frequency for providingsummed eleventh frequency output signal, mixing said tenth frequency andsaid eleventh frequency to provide the first variable output signal fortransmission, and mixing said eighth frequency with said eleventhfrequency for providing the second variable output signal.

9. The method as set forth in claim 8 and further comprising the step ofselectively varying said ninth frequency across a broad high frequencyband for shifting said variable output frequency while maintaining afixed frequency separation therebetween.

10. The method as set forth in claim 9 further comprising the step ofselectively isolating said variable ninth frequency and said sixthfrequency from feedback into adjacent channels during mixing with saideighth, tenth and eleventh frequencies.

1. An exciter frequency agility system comprising: an upconverternetwork having a plurality of input frequencies and first and secondfrequency outputs; a coherent frequency mixer for combining the outputfrequencies of said up-converter and providing a Coherent frequencyoutput; a direction circuit coupled for receiving the coherent output ofsaid frequency mixer and having a plurality of common coherent frequencyoutputs; an isolation and amplifier network having a plurality offrequency inputs and first and second outputs, said outputs havingsimultaneously varying frequencies thereon which remain spaced apart bya fixed frequency difference; said plurality of direction circuitcoherent frequency outputs being coupled to respective isolation networkinputs; a frequency synthesizer having a high frequency variable outputcoupled to a first input of said isolation network; and a second inputof said isolation network being coupled in common with one of saidplural inputs of said converter network.
 2. An exciter frequency agilitysystem as set forth in claim 1 wherein said up-converter networkcomprises first, second and third series coupled mixers, each of saidmixers having first and second inputs and an output; the output of saidfirst mixer being coupled as a first input to said second mixer, theoutput of said second mixer being coupled as a second input to saidthird mixer; the output of said third mixer being coupled as a firstinput to said coherent frequency mixer, said first mixer being coupledto receiver first and second frequencies of said converter plurality ofinput frequencies, said second mixer having the second input coupled toreceive a third frequency of said converter plural input frequencies,said third mixer having the first input coupled to receive a fourthfrequency of said converter plural input frequencies; and a multipliercoupled between a second input of said coherent frequency mixer and saidfourth input frequency of said converter.
 3. An exciter frequencyagility system as set forth in claim 2 and further comprising a hybridhaving an input responsive to said fourth input frequency of saidconverter, said hybrid having a first output coupled to the first inputof said third mixer for coupling said fourth frequency thereto; andwherein said multiplier is a homodyne multiplier having an input coupledto a second output of said hybrid, an output of said multiplier beingcoupled to the second input of said coherent mixer for producing theoutput spectrum of said converter fourth input frequency.
 4. An exciterfrequency agility system as set forth in claim 3 and further comprisinga fixed-tuned filter-amplifier circuit in each of the respective inputsfor said coherent mixer and said third mixer and in the first input ofsaid second mixer for filtering and controlling input signal levelscoupled to said mixer.
 5. A frequency agility system as set forth inclaim 4 wherein said isolator and amplifier network further comprisesthird, fourth and fifth inputs from said common coherent frequency;fourth, fifth, sixth and seventh mixers for selectively combining saidnetwork input frequencies to provide first and second spaced apartvariable output frequencies; said fourth mixer having the output coupledas a first input to said fifth mixer and having first and second inputsrespectively coupled to said fourth frequency as said second input tosaid isolation network and to said third input for providing a fixedoutput frequency difference signal to said fifth mixer, said sixth mixerhaving a first input coupled to said coherent fourth input, said seventhmixer being coupled to said first and fifth network inputs for receivingand mixing said synthesizer variable output signal and said coherentfrequency; a hybrid junction responsive to the output of said seventhmixer for splitting and coupling said variable output signal asrespective second inputs to said fifth and sixth mixers; and first andsecond fixed-tuned bandpass filters coupled respectively to the outputof said fifth mixer and said sixth mixer for coupling said spaced apart,variable output frequencies therefrom.
 6. A frequency agility system asset forth in claim 5 and further comprising respective fixed-tunedfilters and isolators connected Between said hybrid outputs and saidfifth and sixth mixer inputs for selectively isolating the variablefrequency input thereto.
 7. In an exciter frequency agility system, themethod of generating a variable output signal for transmission which isselectively spaced apart from another variable output signal forcombining with the first variable signal to provide a known differencefrequency therebetween and comprising the steps of: mixing first andsecond high frequencies to obtain a third frequency sum output; mixing afourth high frequency with the third frequency to provide a fifthfrequency sum output; mixing a sixth high frequency with the fifthfrequency for providing a seventh high frequency sum output; homodynemultiplying said sixth frequency for producing the fourth harmonic ofsaid sixth frequency; mixing the fourth harmonic of said homodyne sixthfrequency with said seventh frequency for providing an eighth frequency,stable, coherent output signal; and selectively mixing individual onesof said sixth, eighth, and ninth frequencies with mixed combinations ofsaid sixth, eighth and ninth frequencies to provide and selectivelyspaced apart variable output frequencies.
 8. The method as set forth inclaim 7 and wherein said selective mixing comprises the steps of: mixingsaid eighth frequency with said sixth frequency for providing a tenthfrequency output difference signal, mixing said eighth frequency withsaid ninth frequency for providing summed eleventh frequency outputsignal, mixing said tenth frequency and said eleventh frequency toprovide the first variable output signal for transmission, and mixingsaid eighth frequency with said eleventh frequency for providing thesecond variable output signal.
 9. The method as set forth in claim 8 andfurther comprising the step of selectively varying said ninth frequencyacross a broad high frequency band for shifting said variable outputfrequency while maintaining a fixed frequency separation therebetween.10. The method as set forth in claim 9 further comprising the step ofselectively isolating said variable ninth frequency and said sixthfrequency from feedback into adjacent channels during mixing with saideighth, tenth and eleventh frequencies.